Low Temperature Oligomerization of Ethylene over Ni/Al-KIT-6 Catalysts

Article abstract of DOI:10.1007/s10562-017-2036-2

Abstract: In this paper, we have studied the oligomerization of ethylene with a liquid heptane solvent over bifunctional Ni catalysts in a continuous flow reactor. We have prepared an Al-containing KIT-6 silica that was used as a support after calcination in the temperature range of 300–900 °C. The Ni/Al-KIT-6 catalysts had uniform mesopores with diameters in the range of 5.4–6.3 nm, excepting Ni/Al-KIT-6 (900). The calcination temperature of Al-KIT-6 support changed the surface acidity as well as the interaction of Ni²⁺ and acid sites for the Ni catalysts, as determined by temperature-programmed desorption of ammonia, temperature-programmed reduction, infrared spectroscopy after the adsorption of pyridine, solid-state ²⁷Al magic-angle spinning nuclear magnetic resonance spectroscopy, and X-ray adsorption spectroscopy. Among the tested catalysts, the Ni/Al-KIT-6 (300) showed the highest ethylene conversion because of the increased intimate contact between Ni²⁺ and acid sites. The strong interaction of Ni²⁺ species and the support is not effective in increasing active sites for ethylene conversion. The Ni/Al-KIT-6 catalysts produced internal linear C4 and C6 olefins with high selectivity. The Ni/Al-KIT-6 (300) had 2.2–6.1 times lower selectivities toward 2-ethyl-1-butene than other catalysts at similar ethylene conversions. The reaction product mixture showed that the Ni/Al-KIT-6 catalysts shifted the product distribution towards acid-catalyzed oligomerization/cracking/realkylation products (i.e. C3, C7, C7, and C8+ olefins) as the concentration of Br?nsted acid sites increased. Among the tested catalysts, the Ni/Al-KIT-6 (300) showed the highest yield of C4 and C6 olefins (78.3%). Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-017-2036-2

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Catal Lett
DOI 10.1007/s10562-017-2036-2
Low Temperature Oligomerization of Ethylene over Ni/Al-KIT-6
Catalysts
Ahron Hwang1 · Sungtak Kim1 · Geunjae Kwak1,2 · Seok Ki Kim1,2 · Hae-Gu Park1 ·
Seok Chang Kang1 · Ki-Won Jun1,2 · Yong Tae Kim1,2
Received: 1 January 2017 / Accepted: 9 March 2017
© Springer Science+Business Media New York 2017
Abstract In this paper, we have studied the oligomeriza-
tion of ethylene with a liquid heptane solvent over bifunc-
tional Ni catalysts in a continuous flow reactor. We have
prepared an Al-containing KIT-6 silica that was used as a
support after calcination in the temperature range of 300–
900°C. The Ni/Al-KIT-6 catalysts had uniform mesopores
with diameters in the range of 5.4–6.3 nm, excepting Ni/Al-
KIT-6 (900). The calcination temperature of Al-KIT-6 sup-
port changed the surface acidity as well as the interaction
of Ni2+ and acid sites for the Ni catalysts, as determined by
temperature-programmed desorption of ammonia, tempera-
ture-programmed reduction, infrared spectroscopy after the
adsorption of pyridine, solid-state 27Al magic-angle spin-
ning nuclear magnetic resonance spectroscopy, and X-ray
adsorption spectroscopy. Among the tested catalysts, the
Ni/Al-KIT-6 (300) showed the highest ethylene conver-
sion because of the increased intimate contact between
Ni2+ and acid sites. The strong interaction of Ni2+ species
and the support is not effective in increasing active sites for
ethylene conversion. The Ni/Al-KIT-6 catalysts produced
internal linear C4 and C6 olefins with high selectivity.
The Ni/Al-KIT-6 (300) had 2.2–6.1 times lower selectivi-
ties toward 2-ethyl-1-butene than other catalysts at similar
ethylene conversions. The reaction product mixture showed
that the Ni/Al-KIT-6 catalysts shifted the product distri-
bution towards acid-catalyzed oligomerization/cracking/
realkylation products (i.e. C3, C7, C7, and C8+ olefins) as
the concentration of Brønsted acid sites increased. Among
the tested catalysts, the Ni/Al-KIT-6 (300) showed the
highest yield of C4 and C6 olefins (78.3%).
Graphical Abstract
Keywords Ethylene · Oligomerization · Nickel ·
Alumination · KIT-6
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-017-2036-2) contains supplementary
material, which is available to authorized users.
1 Introduction
* Yong Tae Kim
ytkim@krict.re.kr
The potential abundance of shale gas resources has led
to use of a cost-advantaged raw material for the chemical
industry [1]. Light olefins in the range of C2–C4 are cur-
rently produced by steam thermal cracking of hydrocar-
bons (i.e. natural gas liquids or naphtha) [2]. These are
1
Carbon Resources Institute, Korea Research Institute
of Chemical Technology, Daejeon 34114, Republic of Korea
2
Advanced Materials and Chemical Engineering, University
of Science & Technology, Daejeon 34113, Republic of Korea
Vol.:(0123456789)
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A. Hwang et al.
essential building blocks that can be extensively used for
production of liquid fuels, chemical intermediates, or base
materials through the oligomerization process [2–4]. Cost-
effective ethylene in the C2–C4 stream of steam cracking
units is currently receiving great attention recently [5]. For
instance, over several million tonnes of C4-C30+ α-olefins
are currently produced through C–C coupling of ethylene
with homogeneous catalysts developed by several compa-
nies (i.e. Shell (SHOP) and IFP (Dimersol and Alfabutol))
[6–8]. Development efforts are currently focusing on con-
version processes, with a narrow product distribution over
homogeneous catalysts inclusive of transition metal com-
plexes (i.e. Ni, Co, Fe, and Cr) [3, 6, 7]. Immobilization of
homogeneous metal complexes on inorganic supports helps
these materials to be able to separate from oligomer prod-
ucts [9, 10]. In comparison with systems of homogeneous
phase oligomerization, heterogeneous systems have distinct
advantages of recyclability, separation, and tolerance of
impurities.
higher activity compared to their microporous analogs (i.e.
zeolites) because of effective mass transfer inside pores
[49, 50]. Hulea et al. have found that Ni exchanged SBA-15
can serve as a stable, selective, and active catalyst for pro-
duction of C4–C10 olefins from ethylene [39]. They found
that the catalyst has complex dual micro-mesopores with
average pore diameter of 7.9 nm, which is large enough to
allow the free diffusion of oligomer products, leading to a
remarkable activity [39].
The purpose of this study is to understand the catalytic
reaction chemistry on mesoporous silica-alumina sup-
ported Ni catalysts for ethylene oligomerization. We have
employed KIT-6 mesoporous silica as a support; this mate-
rial displays a 3D cubic pore structure with a highly inter-
connected network. KIT-6 mesoporous silica has large
pores with diameter in the range of 4–12 nm, with intra-
wall pores with diameter in the micropore/small mesopore
range [56, 57]. This mesoporous silica may offer a better
dispersion of aluminum species and an enhanced mass
transfer during the reaction. Reaction measurements of Ni/
Al-KIT-6 catalysts were performed to elucidate differences
in the reactivity between these catalysts according to the
calcination temperature of Al-KIT-6 support.
Attempts have been made with solid-acid catalysts,
including zeolites and solid phosphoric acid, to catalyze
olefin oligomerization through a classical carbocation
mechanism [11–15]. The acid sites tend to convert ethyl-
ene at relatively higher temperatures and lower pressures
as compared with other olefins, resulting in production of
gasoline-range hydrocarbons including olefins, paraffins,
aromatics, and cycloalkanes [4]. The acid-based catalysts
are known to minimize undesired side reactions such as
cracking, alkylation, and hydride transfer by decreasing
reaction temperatures below 300°C [12]. Addition of tran-
sition metals (i.e. Ni, Re, W, Co, Fe) to solid-acid catalysts
helps to increase activities of olefin dimerization at mod-
erate temperatures, but higher pressures than those needed
for the solid-acid catalysts [16–23]. Ni supported on solid
acids have been considered as viable candidates for ethyl-
ene oligomerization [24].
2 Experimental
2.1 Catalyst Preparation
Highly ordered mesoporous silica, KIT-6 (SBET =817 m2/g,
total pore volume=1.07 cm3/g), was prepared by sol gel
method using a mixture of (EO)20(PO)70(EO)20 triblock
copolymer (Pluronic P123, Sigma–Aldrich), n-butanol
(Samchun Pure Chemical Co., Ltd.), tetraethyl orthosilicate
(TEOS, Samchun Pure Chemical Co., Ltd.), 35–37% HCl
(Samchun Pure Chemical Co., Ltd.), and deionized water,
as described by Kleitz et al. [56]. The mixture (TEOS:P123
:HCl:H2O:BuOH=1:0.017:1.83:195:1.31) was maintained
at 35°C for 24 h with constant stirring and then aged at
100°C for 24 h in a Teflon lined autoclave. The precipitate
was filtered and washed with deionized water and HCl–eth-
anol solution. It was dried at 110°C overnight, and calcined
in air at 550°C for 4 h.
To optimize the bifunctionality of supported Ni cata-
lysts, various acid supports including alumina [25–30], sil-
ica-alumina [31–45], and zeolite [46–55] have been investi-
gated. Distributions of oligomerized products can be tuned
by adjusting the concentration of Ni cation to the acid sites
as well as by changing the acidity [29–33, 49, 50]. Ni cati-
ons exchanged with H+ in supports are probably responsi-
ble for the initial activation of ethylene through a coordina-
tion insertion mechanism, but NiOx species are inactive in
this reaction [22, 42–44, 46, 47].
Al/KIT-6 was prepared by post synthesis method using
0.012 M NaAlO2 (Kanto Chemical Co., Inc.) aqueous solu-
tion at a molar ratio of Si/Al=16.1, as described by Luan
et al. [58]. The slurry was aged at 30°C for 15 h with con-
stant stirring. The solid was filtered, washed with deionized
water, dried at 80°C overnight, and then calcined in air for
4 h. The calcination temperature in Na-Al/KIT-6 is denoted
as x in Al/KIT-6 (x) (x=300, 500, 700, and 900°C). The
Na-Al/KIT-6 were ion exchanged with 0.5 M NH4NO3
By properly optimizing the porosity of catalysts, it
should be possible to enhance the conversion rate with less
prone to deactivation [41, 53]. Extensive investigations
have been carried out to determine the optimum structural
properties of mesoporous silica-alumina supported Ni cata-
lysts [33, 38–42]. Studies have shown that the mesoporous
forms (i.e. M41S molecular sieves) of Ni catalysts exhibit
1 3

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Low Temperature Oligomerization of Ethylene over Ni/Al-KIT-6 Catalysts
(Kanto Chemical Co., Inc.) aqueous solution at 30°C three
times to obtain the ammonium form.
After sample saturation with pyridine vapors at 30°C for
30 min, infrared spectroscopy (FT-IR) spectra of adsorbed
pyridine were recorded at 30°C using a self-supporting
wafer consisting of 20 mg of each sample; 60 mg of KBr
using a Nicolet 6700 FT-IR spectrometer (Thermo Scien-
tific) with a resolution of 4 cm−1, was placed in an in situ
IR cell. The wafer was dehydrated at 300°C for 2 h prior to
the pyridine adsorption. The excess pyridine was desorbed
by He flushing at 150°C for 18 h before the measurements.
Solid-state 27Al magic angle spinning (MAS) nuclear
magnetic resonance (NMR) spectra were obtained on a
Unity Inova (Varian) device at a resonance frequency of
130.25 MHz. The spectra were acquired using a single
pulse excitation pulse sequence (π/2 pulse) with a 1.0 μs
pulse length and 1 s recycle delay. The 27Al spectra were
calibrated by Al(NO3)2.
Ni/Al-KIT-6 catalysts were prepared by wet impregna-
tion method from NH4-Al/KIT-6 and 0.1 M Ni(NO3)2·6H2O
(Sigma–Aldrich) aqueous solution. The content of Ni was
intended to be fixed at 5 wt%. The Ni/Al-KIT-6 catalysts
were calcined in air at 400°C for 4 h before reaction was
induced.
2.2 Catalyst Characterization
Inductively coupled plasma–atomic emission spectroscopy
(ICP–AES) was conducted to measure the metal loading
for fresh catalysts using an iCAP 6500 (Thermo Scientific).
Using an ASAP 2020 system (Micromeritics), adsorp-
tion and desorption isotherms of nitrogen were obtained
for all of the samples at −196°C. Brunauer–Emmett–Teller
(BET) surface area was calculated on the adsorption
branch. Average pore diameter for each catalyst was cal-
culated by the Barrett-Joyner-Halenda (BJH) desorption
method. Total pore volume was determined at the relative
pressure (P/P0) of 0.99. Before the measurements, the sam-
ple was degassed under vacuum at 300°C for 12 h.
X-ray absorption spectroscopy (XAS) experiments
were performed at the 8C (Nano XAFS) beamline of the
Pohang Light Source (PLS), Korea, in transmittance mode
using gas-ionization detectors. Ni species were calibrated
by simultaneously measuring the spectrum of Ni metal foil.
The collected XAS data were fitted by WinXAS, FEFF,
and IFEF-FIT software. X-ray absorption near edge struc-
ture (XANES) spectra were obtained by normalization.
Extended X-ray absorption fine structure (EXAFS) data
were obtained by performing a Fourier transform.
Bulk crystalline structure of the catalysts was deter-
mined by X-ray diffraction (XRD). The XRD patterns were
obtained with an Ultima IV diffractometer (Rigaku) using
Cu Kα radiation (k=0.15406 nm), operated at 40 kV and
40 mA. Crystal phases were identified using the ICDD
database.
2.3 Catalytic Performance Studies
Ethylene oligomerization was carried out in a stainless-steel
tubular flow reactor, in a downflow arrangement, heated by
a furnace. The reactor was 30 cm long and had an inner
diameter of 8.46 mm. A uniform temperature profile along
the catalyst bed was achieved by using an aluminum heat-
ing block inserted into the void space between the furnace
and the tubular reactor. The temperature was monitored by
a K-type thermocouple in direct contact with the reactor.
For catalytic activity measurements, 2 g of the catalyst with
6 g of SiC were loaded into the reactor. Void spaces on
both ends of the catalyst were filled with SiC (241–559 μm,
Alfa Aesar). The catalyst was pelletized and sieved to a uni-
form diameter of 425–850 μm. Under the reaction condi-
tion, a plug flow patterns with minimizing exothermicity
was ensured. Before the reaction, the pre-calcined catalyst
was pretreated in situ in flowing He (100 mL min−1) at
300°C for 4 h at a heating rate of 5°C min−1 to remove
adsorbed water. After the pretreatment, the furnace was
cooled to 120°C and the reactor system was pressurized
to 4.0 MPa using a backpressure regulator. The olefin feed
(10% C2H4, 1.0% N2 in He) was then fed into the reactor
using mass flow controllers (Brooks Instrument, 5850E).
The liquid solvent (1.3% n-nonane in heptane) was co-fed
into the reactor using a HPLC pump (Younglin, YL 9200,).
After sample saturation with 15 vol% NH3/He at
150°C for 30 min and He flushing for 1.2 h, temperature-
programmed desorption of ammonia (NH3-TPD) was
conducted with 100 mg of each sample in the tempera-
ture range of 150–300°C at a heating rate of 5°C min−1
and under a constant He flow at 30 mL min−1 while the
monitoring thermal conductivity detector (TCD) signals
(Micromeritics, AutoChemII 2920). The sample was held
at 300°C for an additional 2 h. All of the samples were
treated at 500°C for 1 h in He before the experiments to
remove adsorbed water or organic species, except that the
Ni/Al/KIT-6 (300) was treated at 300°C for 1 h.
After sample degassing with He at 300°C for 1 h, tem-
perature-programmed reduction (TPR) was conducted
with 100 mg of each sample in the temperature range of
40–600°C and at a heating rate of 10°C min−1 under a
constant 5% H2/He flow at 30 mL min−1 while monitoring
TCD signals using AutoChemII 2920 unit (Micromerit-
ics). The sample was held at 600°C for an additional 1 h.
A water trap was used to remove moisture from the TPR
effluent stream at −78°C before the TCD. The sample was
purged with 5% H2/He at 40°C for 40 min prior to the TPR
experiments.
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A. Hwang et al.
Nitrogen and n-nonane were used as internal standards.
A chilled 50 mL stainless-steel gas–liquid separator was
used to collect liquid products at the bottom of the reactor
at 10°C and reaction pressure. Another 50 mL gas–liquid
separator was installed after the back-pressure regulator to
condense any residual liquid product at 10°C and 0.1 MPa.
To maintain the reaction pressure, the gaseous products
continued to flow to the back pressure regulator. Online
gas chromatography (GC; Younglin, 6500GC) was used to
analyze the reactor effluent gas. N2 and CH4 in the gaseous
products were analyzed using a thermal conductivity detec-
tor (TCD) with a combination of Porapack Q (Supelco,
Catalog No. 12437) and Molecular sieve 5 Å (Supelco,
Catalog No. 13133-U) packed columns. Both the injec-
tion port and the detector were held at 200°C. The column
flow rate was 20 mL min−1 of He carrier gas. Hydrocar-
bons in the range of C1–C4 components in the gaseous
products were analyzed using a flame ionization detec-
tor (FID) with an Alumina BOND/Na2SO4 (Restek, Cata-
log No. 19756). The injection port and the detector were
held at 200 and 250°C, respectively. The column pressure
was 65.5 kPa with He carrier gas. The following GC oven
temperature regime was used: the temperature was held at
45°C for 2.5 min, ramped to 100°C at 10°C min−1, and
held at 100°C for 5 min. Then, temperature was ramped
to 180°C at 10°C min−1 and held for 30 min. The liquid
product accumulated in the gas–liquid separators was sam-
pled periodically into the online GC at reaction pressure by
using an internal sample injector with a 0.1 μL loop (Valco
Instruments Co. Inc., Catalog No. 2Cl4WE.1). Hydrocar-
bons in the liquid products were analyzed using GC-MS
(Shimadzu, GCMS-QP2010) and online GC (Younglin,
6100GC) with a flame ionization detector that had an Rtx-
DHA (Restek, Catalog. No. 10148) capillary column. For
each identification, a 1 μL liquid sample was injected into
the GC–MS. The injection port and the detector were held
at 200 and 250°C, respectively. The column pressure was
232.4 kPa, with He carrier gas. The following GC oven
temperature regime was used: the temperature was held at
35°C for 5 min, ramped to 50°C at 1.5°C min−1 and held
for 5 min. Temperature was ramped to 200°C at 2°C min−1
and held for 15 min.
The conversion was calculated as moles of feedstock
reacted per moles of feedstock fed to the system. The yield
was calculated by carbon moles of product divided by car-
bon moles of feedstock fed into the system. The selectivity
was calculated as carbon moles of product per total carbon
moles of products detected. The product distribution was
defined as the ratio of carbon moles of product divided by
the total carbon moles of corresponding categories.
3 Results and Discussion
3.1 Catalyst Characterization
Table 1 lists the physicochemical properties of the Ni/
Al-KIT-6 catalysts. The molar ratios of Ni to Al obtained
from ICP analysis were in the range of 1.43–1.99 for the
catalysts. The bulk structure of the Ni/Al-KIT-6 catalysts
was investigated with XRD, with results as shown in Fig. 1.
Except for Ni/Al-KIT-6 (900), the Ni/Al-KIT-6 catalysts
Table 1 Physicochemical properties of Ni/Al-KIT-6 catalysts
Catalysts
Ni
Ni/Al
Primary crystal- SBET (m2/g) Total pore
Average pore Wall
Acid amount
Total (µmol/g)f Brøsted/
content molar
line size of NiO
volume
diameter
thickness
(wt%)a
ratioa
(nm)b
(cm3/g)c
(nm)d
(nm)e
Lewis
ratiog
Ni/Al-KIT-6
(300)
4.71
4.25
4.11
4.19
1.43
1.99
1.73
1.45
9.78
9.23
8.32
12.9
370
328
258
8.01
0.64
0.58
0.42
0.03
6.3
6.0
5.4
9.1
4.9
4.8
4.9
–
443
235
230
–
0.21
0.44
0.48
–
Ni/Al-KIT-6
(500)
Ni/Al-KIT-6
(700)
Ni/Al-KIT-6
(900)
aAmount of Ni and Al was determined by ICP–AES
bPrimary crystalline size was measured using Scherrer equation
cTotal volume was determined at a relative pressure (P/Po) of 0.99
dAverage pore diameter(dP) was determined by the BJH desorption method
√
eWall thickness(d) calculated using d=a0/2−dp, where a0 is the unit cell parameter calculated from a0 =d211
fTotal acidity was determined by quantifying the desorbed NH3 by NH3-TPD
6
gBrønsted/Lewis ratio was determined by FT-IR spectroscopy after pyridine adsorption
1 3